Arrange for a new account to take over existing landline number?
Hi there.
Firstly welcome to our community forums 
I'd actually say you don't need to let us know you want us to keep the line active, because when your family member places an order with us/creates an account at your address, we'll automatically process it just like if we were taking over a line provided by a different provider so it'll take around a minimum of 10 working days to complete.
Upon completion of the new order, your account should be automatically closed and we'll automatically work out your final bill within 21 days which will include any refunds for subscription charges paid in advance for a service you haven't had.
While in many ways this process is similar to migrating between providers, there is one difference. We can't transfer a telephone number between two Plusnet accounts which are in different names. We'll have to provide a new phone number.
That said, once the order has completed, assuming the phone number is owned by BT (You can check Here, type your number in and click Lookup Number then check the Operator, we should be able to easily renumber the line within 1 working day if we are able to grab the number before it's allocated to another line. Generally a renumber request costs £36.63.
Some numbers however are owned by rangeholders/operators like Sky, Talk Talk and Virgin Media to name a few, and once the number is made inactive it goes back to them, we'd struggle to import an inactive number from a non-BT rangeholder.
A line installation fee shouldn't be required at all, however an activation fee may be required depending on what offer they sign up on. Looking Here currently we've discounted the activation fee to nothing from £10.
To try to ensure things go smoothly, I would recommend them not* advising us of the existing phone number during the beginning of the signup process online (where you check for availability). Our system uses this when placing an order with our suppliers and it'll stumble into a problem if it tries to place an order with a number that's already on an existing account.
*Just click "I don't have a phone number" and proceed based on the address.
If they sign up over the phone, it may be best not telling the adviser about the existing number as when we sign new customers up ourselves we'd go through a very similar process like you do online through our website.
Hope this helps. Let me know if you need any further assistance or anything clarifying, feel free to also let us know if you/they decide to go ahead and we'll be happy to keep an eye on things.
Hi Hil. Cheers for getting back to me.
I've had a look into both yours and their accounts and the order is progressing fine. Looks like we've sent both you and them notification of the order in progress that's due to complete on the 20/06/2019.
I'd just like to note that we may not be able to get the telephone number back once the order has completed, however there's a good chance we can though. Just to let you know in case it's absolutely imperative they keep the same number.
I've just created a support ticket on their account which they can view and reply to from here: https://www.plus.net/wizard/?p=view_question&id=190951967 after they've logged in. To keep things documented on the account, could you ask them to confirm the telephone number and also that they're happy to go ahead please?
Cheers again.